Thursday, March 18, 2010

Valve adjustment and TM-40 carburetor tuning

I'm off work this week minding the kids while they're on March break. While they were busy riding their bicycles with the all the neighbourhood brats, I took some time to tinker with the bike.

When I had the engine all apart over the winter, I had intended to check the valve clearances, however, I wasn't able to locate my feeler gauges at the time. As Murphy's Law would dictate, I only managed to find the gauge set after I finished buttoning up the entire engine. Anyway, today was a  nice day so I decided it was time to get it done. Of the two intake and two exhaust valves, only one intake was out of spec -- and not by much.

Since that didn't take too long, I wanted to make another little change to my new TM-40 carburetor. It's a great carb -- way better than the factory CV unit (although I did have that one set up and running quite well with the intake system mods and DynoJet kit). Anyway, the new TM-40 was working great but it seemed a little rich in the first 1/2 throttle. The spark plugs and muffler outlet pipe were also a little black and coated with soot. After having swapped the pilot jet for a size smaller (from a size 22.5 to a 20) and fiddled around with the pilot air/fuel mixture screw last week, it seemed to help in some ways but it wasn't perfect: it seemed less responsive than it should. With the smaller pilot jet, the spark plugs looked a little lean (white) on the electrode, yet sooty on the outside edges of the plug, near the threads.

So, with the gas tank, seat, and side covers removed for the valve adjustment, I swapped back to the original pilot jet, and moved the needle clip up one notch (from the 3rd clip position to the 2nd). This effectively lowers the needle into the needle jet down inside the bowl, leaning the mixture but only in the 1/4-1/2 throttle range opening. From there, I turned the pilot screw in about 1/2 a turn to lean out the idle mixture.

After a good hard ride around the city, plus a few Kilometres on the highway, it now pulls hard right off idle to wide open throttle. It's also very responsive. That seemed to be the missing link. After cruising a Km or two in 3rd gear with it revving pretty good (around 85Km/hr) I pulled over on the side of the road and quickly removed a spark plug to check. A nice even tan brown colour. Awesome.

I expect that fuel economy will improve due to the needle adjustment, although I haven't been keeping track of it while I iron out the little bugs. With the BST carb, I was averaging 150 Km of  hard riding in the city for roughly 10 Litres of fuel. That's roughly 35 mpg -- not so great, but I hammer it pretty hard most of the time. I'm curious to find out if that will change.